The human heart normally maintains its own well-ordered intrinsic rhythm through generation of stimuli by pacemaker tissue that results in a wave of depolarization that spreads through specialized conducting tissue and then into and through the myocardium. The well-ordered propagation of electrical depolarizations through the heart causes coordinated contractions of the myocardium that results in the efficient pumping of blood. In a normally functioning heart, stimuli are generated under the influence of various physiological regulatory mechanisms to cause the heart to beat at a rate that maintains cardiac output at a level sufficient to meet the metabolic needs of the body. Abnormalities of excitable cardiac tissue, however, can lead to abnormalities of heart rhythm that are called arrhythmias. All arrhythmias stem from one of two causes: abnormalities of impulse generation or abnormalities of impulse propagation. Arrhythmias can cause the heart to beat too slowly (bradycardia, or a bradyarrhythmia) or too quickly (tachycardia, or a tachyarrhythmia), either of which may cause hemodynamic compromise or death.
Drug therapy is often effective in preventing the development of arrhythmias and in restoring normal heart rhythms once an arrhythmia has occurred. However, drug therapy is not always effective for treating particular arrhythmias, and drug therapy usually causes side-effects that may be intolerable in certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system incorporated into an implantable device that delivers therapy to the heart in the form of electrical stimuli. Such implantable devices include cardiac pacemakers that deliver timed sequences of low energy electrical stimuli, called pacing pulses, to the heart, via an intravascular leadwire or catheter (referred to as a lead) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pacing pulses (referred to as capturing the heart). By properly timing the delivery of pacing pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacemakers are often used to treat patients with bradycardia. Pacemakers are also capable of delivering paces to the heart in such a manner that the heart rate is slowed, a pacing mode referred to as anti-tachyarrhythmia pacing.
Cardiac rhythm management systems also include cardioverter/defibrillators (ICD's) that are capable of delivering higher energy electrical stimuli to the heart. ICD's are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Tachyarrhythmias can cause diminished blood circulation because the cardiac cycle of systole (contraction) and diastole (filling) can be shortened to such an extent that insufficient blood fills the ventricles during diastole. Besides the potential for such hemodynamic embarrassment, tachyarrhythmias can also degrade into even more serious arrhythmias such as fibrillation where electrical activity spreads through the myocardium in a disorganized fashion so that effective contraction does not occur. For example, in a particular type of tachyarrhythmia, referred to as ventricular fibrillation, the heart pumps little or no blood to the body so that death occurs within minutes. A defibrillator delivers a high energy electrical stimulus or shock to the heart to depolarize all of the myocardium and render it refractory in order to terminate arrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to ICD's and pacemakers, cardiac rhythm management systems also include pacemaker/ICD's that combine the functions of pacemakers and ICD's, drug delivery devices, and any other implantable or external systems or devices for diagnosing, monitoring, or treating cardiac arrhythmias.
Cardiac rhythm management systems incorporated into ICD's allow tachyarrhythmias to be automatically detected and treated in a matter of seconds. Defibrillators are usually effective at treating tachyarrhythmias and preventing death, but such devices are not 100% effective at treating all tachyarrhythmias in all patients. As a result, some patients may still die even if the defibrillator delivers appropriate therapy. Also, some patients have frequent tachyarrhythmias, triggering frequent therapeutic shocks. This reduces the usable life of the implanted battery-powered device and increases the risk of therapy-induced complications. Furthermore, even if the device successfully treats the tachyarrhythmia, the patient may lose consciousness during the arrhythmia which can result in related serious or even fatal injuries (e.g., falling, drowning while bathing, car accident while driving, etc.). Thus, there is a need for a cardiac rhythm management system that predicts when an arrhythmia will occur and invokes a therapy to prevent or reduce the consequences of the arrhythmia.